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Model
R-874 (serv.man5)
Pages
45
Size
1.38 MB
Type
PDF
Document
Service Manual
Brand
Device
Home Appliance / Microwave Oven / Complete
File
r-874-sm5.pdf
Date

Sharp R-874 (serv.man5) Service Manual ▷ View online

R-874M - 6
OPERATION SEQUENCE
OFF CONDITION
Closing the door activates the monitored latch switch and
the stop switch.
IMPORTANT:
When the oven door is closed, the contacts COM-NC
of the monitor switch  must be open. When the micro-
wave oven is plugged in a wall outlet (230 - 240V /
50Hz), the line voltage is supplied to the noise filter.
Figure O-1 on page
 30
1. The control unit is not energized. The display shows
nothing (Fig. O-1 (a)).
2. Open the door. The contacts (COM-NC) of the moni-
tored latch switch are closed and the control unit is
energized. Then contacts of relays RY1 and RY5 are
closed, and the oven lamp will light and the display will
show "ENERGY SAVE MODE" (Fig. O-1(b)).
3. Close the door. The contacts (COM-NC) of the moni-
tored latch switch are opened and the contacts of relay
RY1 are opened and the oven lamp will be turned off.
The display will show "  .  
0". (Fig. O-1(c)).
NOTE: Energy save mode
1. If the oven has not been used for more than 2 minutes,
the contacts of the relay RY5 will be opened and the
control unit will be not energized. Open and close the
door, the control unit will resume.
2. If the clock is set, this energy save mode does not work.
MICROWAVE COOKING CONDITION
HIGH COOKING
Enter a desired cooking time by touching the TIME keys
and start the oven by touching START key.
Function sequence Figure O-2 on page 31
CONNECTED COMPONENTS
RELAY
Oven lamp, Turntable motor
RY1
High voltage transformer
RY2
Fan motor
RY6
1. The line voltage is supplied to the primary winding of
the high voltage transformer. The voltage is converted
to about 3.3 volts A.C. output on the filament winding
and high voltage of approximately 2000 volts A.C. on
the secondary winding.
2. The filament winding voltage (3.3 volts) heats the
magnetron filament and the high voltage (2000 volts) is
sent to the voltage doubling circuit, where it is doubled
to negative voltage of approximately 4000 volts D.C..
3. The 2450 MHz microwave energy produced in the
magnetron generates a wavelength of 12.24 cm. This
energy is channelled through the waveguide (transport
channel) into the oven cavity, where the food is placed
to be cooked.
4. When the cooking time is up, a signal tone is heard and
the relays RY1 + RY2 + RY6 go back to their home
position. The circuits to the oven lamp, high voltage
transformer, fan motor and turntable motor are cut off.
5. When the oven door is opened during a cooking cycle,
the switches come to the following condition.
Switch
Contact
          Condition
During
Oven Door
Cooking
Open(No cooking)
COM-NO
Closed
Opened
Monitored latch switch
COM-NC
Opened
Closed
Stop switch
COM-NO
Closed
Opened
COM-NO
Closed
Opened
Monitor Switch
COM-NC
Opened
Closed
The circuit to the high voltage transformer is cut off when
the contacts of relay RY2, and the contacts (COM-NO) of
the monitored latch switch and monitor switch are made
open. The circuit to the fan motor is cut off when the relay
RY6 is made open. The circuit to the turntable motor is cut
off when the contacts (COM-NO) of the monitored latch
switch are made open. The relay RY2 and RY6 are made
open when the door is opened. The oven lamp remains on
even if the oven door is opened after the cooking cycle has
been interrupted, because the relay RY1  stays closed.
Shown in the display is remaining time.
6. MONITOR SWITCH CIRCUIT
The monitor switch is mechanically controlled by the
oven door, and monitors the operation of the moni-
tored latch switch.
6-1. When the oven door is opened during or after the
cycle of a cooking program, the monitored latch
switch and stop switch must open their contacts
(COM-NO) first. And the contacts (COM-NC) of the
monitored latch switch are made closed. After that the
contacts (COM-NC) of the monitor switch can be
closed and the contacts (COM-NO) of monitor switch
are made open.
6-2. When the oven door is closed, the contacts (COM-
NC) of the monitor switch SW3 must be opened and
the contacts (COM-NO) of monitor switch must be
closed. After that the contacts (COM-NO)  of the
monitored latch switch and the stop switch are made
closed. And the contacts (COM-NC) of the monitored
latch switch are made open.
6-3. When the oven door is opened and the contacts
(COM-NO) of  the monitored latch switch remain
closed, the fuse F8A will blow. Because the relay RY1
and monitor switch are closed  and a short circuit is
caused.
MEDIUM HIGH, MEDIUM, MEDIUM LOW, LOW
COOKING
When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the high voltage
transformer intermittently within a 32-second time base
through the relay contact which is coupled with the cur-
rent-limiting relay RY2. The following levels of microwave
power are given.
Note:
The On/Off time ratio does not exactly correspond
to the percentage of microwave power, because
approx. 3 seconds  are needed for heating up the
magnetron filament.
GRILL COOKING CONDITION
TOP GRILL (Figure O-3a)
In this condition the food is cooked by the top grill heating
element . Programme the desired cooking time by touch-
ing the TIME keys and touh the GRILL key once. When the
STARTkey is touched, the following operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY3 is energized and the main supply
900W
(HIGH)
32 sec. ON
630W  MEDIUM 
HIGH 
Approx. 70%
100%
24 sec. ON
8 sec. OFF
14 sec. OFF
20 sec. OFF
26 sec. OFF
450W (MEDIUM)
Approx. 50%
18 sec. ON
270W  MEDIUM 
LOW 
Approx. 30%
12 sec. ON
90W
(LOW)
Approx. 10%
6 sec. ON
R-874 S/MANUAL
11/7/01, 10:59 am
6
R-874M - 7
voltage is applied to the top grill heating element .
4. Now, the food is cooked by the top grill heating ele-
ment.
BOTTOM GRILL (Figure O-3b)
In this condition the food is cooked by bottom grill heating
element energy. Programme the desired cooking time by
touching the TIME keys and touh the GRILL key twice.
When the STARTkey is touched, the following operations
occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY4 is energized and the main supply
voltage is applied to the bottom grill heating element .
4. Now, the food is cooked by the bottom grill heating
element .
TOP AND BOTTOM GRILL (Figure O-3c)
In this condition the food is cooked by top and bottom grill
heating elements energy. Programme the desired cook-
ing time by touching the TIME keys and touh the GRILL
key three times. When the STARTkey is touched, the
following operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY4 is energized and the main supply
voltage is applied to the bottom grill heating element.
4. The relay RY3 is energized and the main supply
voltage is applied to the top grill heating element.
5. Now, the food is cooked by the top and bottom grill
heating elements.
CONVECTION COOKING CONDITION
PRE-HEATING (Figure O-4)
Programme the desired convection temperature by touch-
ing CONVECTION TEMPERATURE key. When the
STARTkey is touched, the following operations occur:
1. The coil shut-off relays RY1  and RY6 are energized,
the oven lamp, cooling fan motor and turntable motor
are turned on.
2. The coil shut-off relays RY3  RY 7 and RY4 are
energized by control unit and the main supply voltage
is added to the top and bottom heating elements and
covection motor.
3. When the oven temperature reaches the selected pre-
heat temperature,  the following operations occur:
3-1. The coil shut-off relays RY3  and RY4 are energized
by control unit temperature circuit and thermistor,
opening the circuit to the top and bottom grill heating
elements and convection motor.
3-2. The oven will continue to function for 30 minutes,
turning the top and bottom heating elements on and
off, as needed to maintain the selected pre-heat
temperature. The ovenwill shut-down completely af-
ter 30 minutes.
NOTE: The fan motor will be turned on and off at
preheating. And the convection motor will be turned
off at  preheating. See "Power setting" on page 9.
CONVECTION COOKING (Figure O-4)
When the pre-heat temperature is reached, a beep signal
will sound indicating that the holding the temperature has
been reached in the oven cavity. Open the door and place
the food to cooked in the oven. Program desired cooking
time by touching the TIME keys and program convection
temperature by touching the CONVECTION TEMPERA-
TURE key . When the START key is touched, the following
operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor, turntable motor and
convection motor are energized.
NOTE: When the convection temperature 40˚C is se-
lected, the convection motor is not energized.
3. The relays RY3  and RY4 are energized (if the cavity
temperature is lower than selected temperature) and
the main supply voltage is applied to the top and
bottom heating elements to return to the selected
cooking temperature.
4. Upon completion of the cooking time, the audible sig-
nal will sound, and the oven lamp, turntable motor,
cooling fan motor, convection motor, top and bottom
heating elements are de-energized. At the end of con-
vection cycle, if the cavity air temperature rise above
120˚C, the circuit to the relay RY6 will be maintained
(by the thermistor circuit) to continue operation of the
cooling fan motor until temperature drops below 105˚C,
at that time the relay RY6 will be de-energized, turning
off the fan motor.
DUAL COOKING CONDITION
MICROWAVE AND CONVECTION  (Figure O-5a)
Programme the desired cooking time by touching the
TIME keys. Touch the DUAL CONVECTION/GRILL key
once. Select the microwave power level by touching the
MICROWAVE POWER LEVEL key. And select the con-
vection temperature by touching the CONVECTION TEM-
PERATURE key.
NOTE: The microwave power level can be selected from
90W, 270W and 450W.
When the STARTkey is touched, the following operations
occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor, turntable motor and
convection motor are energized.
3. The relay RY3 will be energized and the main supply
voltage is applied to the top grill heatier.
4. The relay RY4 is energized (if the cavity temperature is
lower than selected temperature) and the main supply
voltage is applied to the bottom heating elementer.
5. The relay RY2 is energized and the microwave energy
is generated by magnetron.
6. Now, the food is cooked by microwave and convection
energy simultaneously.
MICROWAVE AND TOP GRILL  (Figure O-5b)
Programme the desired cooking time by touching the
TIME keys. Touch the DUAL CONVECTION/GRILLL key
twice. Select the microwave power level by touching the
MICROWAVE POWER LEVEL key. When the STARTkey
is touched, the following operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY3 is energized and the main supply
voltage is applied to the top grill heating element.
4. The relay RY2 is energized and the microwave energy
is generated by magnetron.
5. Now, the food is cooked by microwave and top grill
OPERATION SEQUENCE
R-874 S/MANUAL
11/7/01, 10:59 am
7
R-874M - 8
simultaneously.
MICROWAVE AND BOTTOM GRILL  (Figure O-5c)
Programme the desired cooking time by touching the
TIME keys. Touch the DUAL CONVECTION/GRILL three
times. Select the microwave power level by touching the
MICROWAVE POWER LEVEL key. When the STARTkey
is touched, the following operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY4 is energized and the main supply
voltage is applied to the bottom grill heating element.
4. The relay RY2 is energized and the microwave energy
is generated by magnetron.
5. Now, the food is cooked by microwave and bottom grill
simultaneously.
POWER SETTING
Following table shows each power setting of the top
heating element, bottom heating element, magnetron, fan
motor and convection motor on Convection cooking mode,
Dual (Micro + Convec.) mode and Convection preheating
mode.
POWER OUTPUT
ON TIME
OFF TIME
100%
48 sec.
0 sec.
90%
44 sec.
4 sec.
80%
40 sec.
8 sec.
70%
36 sec.
12 sec.
60%
32 sec.
16 sec.
50%
26 sec.
22 sec.
40%
22 sec.
26 sec.
30%
16 sec.
32 sec.
20%
12 sec.
36 sec.
10%
8 sec.
40 sec.
AUTOMATIC COOKING
PIZZA
BREAKFAST
INSTANT COOK
AUTO COOK
AUTO DEFROST
Above functions are automatic cooking. They automati-
cally work out the correct cooking mode and cooking time
and/or cooking temperature. They will cook according to
the special cooking sequence.
LIMITATIONS OF POWER OUTPUT IN MANUAL
OPERATION
After the same cooking mode is carried out for more than
the specified cooking time, the power output is automati-
cally reduced by turning the control relays on and off
intermittently, as shown in the table below. This is to
protect the oven door against temperature rising.
OPERATION SEQUENCE
CONV. COOKING
DUAL/CONVEC. COOKING
CONVE
TOP
BOTTOM
TOP
BOTTOM
MAGNETRON
FAN
CTION
HEATER
HEATER
HEATER
HEATER
(MICRO)
MOTOR
MOTOR
250˚C
70%
70%
70%
70%
10, 30, 50%
100%
100%
230˚C
60%
70%
50%
70%
10, 30, 50%
100%
100%
220˚C
40%
70%
40%
70%
10, 30, 50%
100%
100%
200˚C
40%
60%
40%
60%
10, 30, 50%
100%
100%
180˚C
40%
50%
40%
50%
10, 30, 50%
100%
100%
160˚C
20%
40%
20%
40%
10, 30, 50%
100%
100%
130˚C
20%
30%
20%
30%
10, 30, 50%
100%
100%
100˚C
20%
20%
20%
20%
10, 30, 50%
100%
100%
70˚C
10%
20%
10%
20%
10, 30, 50%
100%
100%
40˚C
10%
10%
10%
10%
10, 30, 50%
100%
0%
CONVECTION PREHEATING
TOP HEATER
BOTTOM HEATER
FAN MOTOR
CONV. MOTOR
250˚C
100%
100%
60%
0%
230˚C
100%
100%
60%
0%
220˚C
100%
100%
60%
0%
200˚C
100%
100%
60%
0%
180˚C
100%
100%
60%
0%
160˚C
100%
100%
60%
0%
130˚C
100%
100%
60%
0%
100˚C
100%
100%
60%
0%
70˚C
100%
100%
60%
0%
40˚C
100%
100%
60%
0%
ON/OFF TIME RATIO
In grill cooking, convection cooking or dual cooking, the
top heater, bottom heater or magnetron operate whithin a
48 second time base. The following table is the ON / OFF
time ratio at each power output of the top heaters, bottom
heater or magnetron.
D
U
A
L
Specified cooking
Limited power
Time base
Cooking mode
time (minutes)
output (%)
(seconds)
Microwave (100%)
20
70
32
Top grill
30
50
48
Bottom grill
45
50
48
Top grill and
15 (Top)
50
48
Bottom grill
15(Bottom)
50
48
Micro. (100%)
20 (Micro.)
70
48
+ Top grill
15 (Grill)
50
48
Micro. (100%)
20 (Micro.)
70
48
+ Bottom grill
15 (Heater)
50
48
NOTE:
1. In case of Automatic operations, the limitations of
power output are not carried out.
2. In case that the stop key is touched or the oven door
is opened during cooking, the limitations of power
output are not carried out after the total cooking time
beyond the specified cooking time.
3. In case of the two or more same cooking modes  are
carried out,  the limitations of power output are not
carried out after the total cooking time beyond the
specified cooking time.
4. In case of the two or more different cooking modes  are
carried out,  the specified cooking time is started to
count from the point when the cooking mode is changed.
5. If the cooking mode has the power level display, the
power level is also displayed when the limitations of
R-874 S/MANUAL
11/7/01, 10:59 am
8
R-874M - 9
DOOR OPEN MECHANISM
The door can be opened by pushing the open button on the
control panel. When the open button is pushed, the open
lever pushes the lower latch head on the door upward.
The upper latch head is linked with the  lower latch head,
so now, the door can be opened.
Figure D-1. Door Open Mechanism
MONITORED LATCH SWITCH
1. When the oven door is closed, the contacts (COM-NO)
of the switch must be closed. And the contacts (COM-
NC) must be opened.
2. When the oven door is opened, the contacts (COM-
NO) of the switch must be opened. And the contacts
(COM-NC) must be closed.
STOP SWITCH
1. When the oven door is closed, the contacts (COM-NO)
of the switch must be closed.
2. When the oven door is opened, the contacts (COM-
NO) of switch must be opened.
MONITOR SWITCH
The monitor switch is activated (the contacts opened) by the
upper latch head on the door while the door is closed. The
switch is intended to render the oven inoperative by means of
blowing the fuse F8A when the contacts of the monitored
latch switch fail to open when the door is opened.
Function
1. When the door is opened, the contacts (COM-NC) of
monitor switch  close (to the ON condition) due to their
being normally closed and contacts (COM-NO) open.
At this time the contacts (COM-NO) of monitored latch
switch is in the OFF condition (contacts open) due to
their being normally open contact switches.
2. As the door goes to a closed position, the monitor
switch contacts (COM-NC) are opened and contacts
(COM-NO) closed and then contacts (COM-NO) of
monitored latch switch and stop switch are closed.(On
opening the door, each of these switches operate
inversely.)
3. If the door is opened and the monitored latch switch
contacts  (COM-NO) fail to open, the fuse F8A blows
immediately after closing of the monitor switch (COM-
NC) contacts.
CAUTION: BEFORE REPLACING A BLOWN FUSE F8A,
TEST THE MONITORED LATCH SWITCH
AND  MONITOR SWITCH FOR PROPER
OPERATION.  (REFER TO CHAPTER “TEST
PROCEDURE”).
FUSE F1 20A 250V
If the wire harness or electrical components are short-
circuited, this fuse blows to prevent an electric shock or fire
hazard.
FUSE F2 F8A 250V
1. If the wire harness or electrical components are short-
circuited, this fuse blows to prevent an electric shock or
fire hazard.
2. The fuse also blows when the monitored latch switch
remains closed with the oven door open and when the
monitor switch contact (COM-NC) closes.
3. The fuse also blows when the asymmetric rectifier,
H.V. rectifier,.H.V. wire harness, H.V. capacitor,
magnetron or secondary winding of high voltage trans-
former is shorted.
TC TRANSFORMER
T/C transformer converts A.C. line voltage into low voltage
to drive the control unit.
THERMAL CUT-OUT 125˚C  (MG)
This thermal cut-out protects the magnetron against over-
heat. If the temperature goes up higher than 125˚C be-
cause the fan motor is interrupted or the ventilation open-
ings are blocked, the thermal cut-out will open and line
voltage to the high voltage transformer will cut off and
operation of the magnetron will be stopped. The defec-
tive thermal cut-out must be replaced with a new one.
THERMAL CUT-OUT 150˚C (OVEN)
This thermal cut-out protects the oven against the over-
heat during grill cooking, convection cooking or dual
(combination) cooking. If the temperature rises above
150˚C because the fan motor is interrupted, the air inlet
duct is blocked or the ventilation openings are obstructed,
the thermal cut-out opens and switches off all the electrical
parts. When the cools itself down to the operating tem-
perature of 130˚C, the contacts of the thermal cut-out will
close again.
ASYMMETRIC RECTIFIER
The asymmetric rectifier is a solid state device that pre-
vents current flow in both directions.  It prevents the
overheating of the high voltage transformer by blowing the
fuseF8A when the high voltage rectifier is shorted.
The rated peak reverse voltage of D1 of the asymmetric
rectifier is 6 KV. The rated peak reverse voltage of D2 of
the asymmetric rectifier is 1.7 KV. D1 and D2 of the
asymmetric rectifier or high voltage rectifier are shorted
when the each peak reverse voltage goes beyond the
each rated peak reverse voltage. (The process of the
blowing the fuse F8A.)
FUNCTION OF IMPORTANT COMPONENTS
D2
D1
ASYMMETRIC
RECTIFIER
HIGH VOLTAGE 
RECTIFIER
1. The high voltage rectifier is shorted by some fault when
microwave cooking or dual cooking.
2. The peak reverse voltage of D2 of the rectifier goes
beyond the rated peak reverse voltage 1.7 KV in the
voltage doubler circuit.
3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high volt-
age winding of the high voltage transformer.
5. The large electric currents beyond 8A flow through the
DOOR
LATCH HOOK
MONITOR SWITCH
MONITORED
LATCH SWITCH
 STOP SWITCH
OPEN LEVER
LATCH
HEADS
R-874 S/MANUAL
11/7/01, 10:59 am
9
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